Sub-lithographic gate length transistor using self-assembling polymers

ABSTRACT

A semiconductor structure including at least one transistor located on a surface of a semiconductor substrate, wherein the at least one transistor has a sub-lithographic channel length, is provided. Also provided is a method to form such a semiconductor structure using self-assembling block copolymer that can be placed at a specific location using a pre-fabricated hard mask pattern.

FIELD OF THE INVENTION

The present invention relates to semiconductor device manufacturing, andmore particularly to a method of forming a transistor having asub-lithographic gate length using self-assembling copolymers. Thepresent invention also relates to a semiconductor structure including atleast one transistor having a sub-lithographic gate length.

BACKGROUND OF THE INVENTION

Over the past twenty-five years or so, the primary challenge of verylarge scale integration (VLSI) has been the integration of anever-increasing number of metal oxide semiconductor field effecttransistor (MOSFET) devices with high yield and reliability. This wasachieved mainly in the prior art by scaling down the MOSFET channellength without excessive short-channel effects. As is known to thoseskilled in the art, short-channel effects are the decrease in thresholdvoltage Vt in short-channel devices due to two-dimensional electrostaticcharge sharing between the gate and the source/drain diffusions.

As the 45 nm node and the 32 nm node generations of complementarymetal-oxide-semiconductor (CMOS) devices are approached, scaling of atransistor's gate length becomes imperative. However, the scaling effortis significantly limited by conventional lithographic printing, whichhas been used for patterning the gates of such transistors. The current0.93 NA (numerical aperture) lithographic tool used in patterning thegates can only resolve 90 nm line width. Future 1.2 NA immersionlithographic tools are expected to print 70 nm line widths. As such,there is a need to reduce the gate length of transistors below 60,preferably below 50, nm.

Moreover, the line edge roughness and critical dimension variation ofconventional lithography results in more significant Vt variation due tosmaller feature size.

In view of the above, there is a need to provide a method of scalingsemiconductor transistors to have sub-lithographic (less than 60,preferably less than about 50, nm) gate lengths.

SUMMARY OF THE INVENTION

The present invention provides a semiconductor structure having at leastone transistor located on a surface of a semiconductor surface, whereinthe at least one transistor has a sub-lithographic channel length. Thepresent invention also provides a method to form such a semiconductorstructure using a self-assembling block copolymer that can be placed ata specific location using a pre-fabricated hard mask pattern. Theinventive method provides a semiconductor structure that hassub-lithographic channel lengths. It is noted that the term“sub-lithographic” is used throughout the instant application to denotedimensions, including channel lengths, below 60 nm.

In one aspect of the present invention, a semiconductor structure isprovided that comprises:

at least one transistor located on a surface of a semiconductorsubstrate, said at least one transistor having a gate length of lessthan 60 nm and a line edge roughness in terms of 3-sigma of less than 6nm.

Line edge roughness (LER) from conventional lithography illumination andphotoresist processing leads to device parameter fluctuation and devicemismatch, resulting in reduced process window and product performancedegradation. As device dimension scales, the impact of roughnessincreases. LER can be measured statistically using a variabilityparameter called variance or ‘sigma’. The 3-sigma of conventionalpatterned gate stacks is greater than 6 nm.

In an embodiment of the present invention, the at least one transistoris a p-type field effect transistor (pFET). In another embodiment of thepresent invention, the at least one transistor is an n-type field effecttransistor (nFET). In yet another embodiment of the present invention,the structure includes at least one pFET and a least one nFET, whereinboth types of transistors have sub-lithographic channel lengths.

In another aspect of the present invention, a method of fabricating asemiconductor structure including at least one transistor having asub-lithographic channel length located on a surface of a semiconductorsubstrate is provided. The method of the present invention combinesconventional lithographic technology and a copolymer self-assemblingtechnique to place one polymer unit with precise position. The polymeris self-aligned to the pre-defined lithographically defined patternboundaries. The single polymer can then be selectively removed and theremaining polymer is used as a photo mask to pattern a transistor gate.The final feature size is determined by the polymer molecule rather thanconventional lithography. Because conventional lithography is not usedin providing the final gate transistor, defects such as line edgeroughness and CD variation are avoided.

In general terms, the method of the present invention comprises:

providing a structure including a patterned hard mask located above asacrificial layer and a semiconductor substrate, wherein said patternedhard mask includes at least one lithographically defined openingexposing a surface of said sacrificial layer;

providing a block copolymer having a single unit polymer block with asub-lithographic width inside the at least one lithographically definedopening, wherein the single unit polymer block comprises a secondpolymeric block component which is embedded in a polymeric matrix thatcomprises a first polymeric block component of said block copolymer;

selectively removing the second polymeric block component relative tothe first polymeric block component to form a sub-lithographic openingin the polymeric matrix inside the at least one lithographically definedopening;

transferring said sub-lithographic opening to said sacrificial layer;

removing the block copolymer and the patterned hard mask;

forming a gate dielectric and a gate conductor within said at least onelithographical defined opening; and

removing the sacrificial layer.

The block copolymers as mentioned hereinabove preferably comprise A:Bblock copolymers wherein A is a first polymeric component and B is asecond polymeric component having a weight ratio of A:B from about 20:80to about 80:20, and the single unit polymer block preferably comprises acylinder that stands vertical to an upper surface of the semiconductorsubstrate. The block copolymers can be readily selected from the groupconsisting of polystyrene-block-polymethylmethacrylate (PS-b-PMMA),polystyrene-block-polyisoprene (PS-b-PI),polystyrene-block-polybutadiene (PS-b-PBD),polystyrene-block-polyvinylpyridine (PS-b-PVP),polystyrene-block-polyethyleneoxide (PS-b-PEO),polystyrene-block-polyethylene (PS-b-PE),polystyrene-b-polyorganosilicate (PS-b-POS),polystyrene-block-polyferrocenyldimethylsilane (PS-b-PFS),polyethyleneoxide-block-polyisoprene (PEO-b-PI),polyethyleneoxide-block-polybutadiene (PEO-b-PBD),polyethyleneoxide-block-polymethylmethacrylate (PEO-b-PMMA),polyethyleneoxide-block-polyethylethylene (PEO-b-PEE),polybutadiene-block-polyvinylpyridine (PBD-b-PVP), andpolyisoprene-block-polymethylmethacrylate (PI-b-PMMA). In a particularlypreferred, but not necessary, embodiment of the present invention, theblock copolymers comprise PS-b-PMMA having a PS:PMMA weight ratioranging from about 20:80 to about 80:20.

Other aspects, features and advantages of the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B are pictorial representations (through a top-down and across sectional view) of a structure which includes a patterned hardmask having at least one lithographically defined opening located atop asacrificial layer and a semiconductor substrate.

FIGS. 2A-2B are pictorial representations (through a top-down and across sectional view) of the structure of FIGS. 1A-1B after applying aself-assembling copolymer thereto.

FIGS. 3A-3B are pictorial representations (through a top-down and across sectional view) of the structure of FIGS. 2A-2B after annealingthe self-assembling copolymer.

FIGS. 4A-4B are pictorial representations (through a top-down and across sectional view) of the structure of FIGS. 3A-3B after selectivelyremoving one of the polymeric components of the self-assemblingcopolymer forming at least one sub-lithographic opening in saidcopolymer.

FIGS. 5A-5B are pictorial representations (through a top-down and across sectional view) of the structure of FIGS. 4A-4B after extendingthe sub-lithographic opening through the sacrificial layer.

FIGS. 6A-6B are pictorial representations (through a top-down and across sectional view) of the structure of FIGS. 6A-6B after strippingthe copolymer and the patterned hard mask.

FIGS. 7A-7B are pictorial representations (through a top-down and across sectional view) of the structure of FIGS. 6A-6B after forming agate dielectric on an exposed surface of the semiconductor substratethrough the extended sub-lithographic opening.

FIGS. 8A-8B are pictorial representations (through a top-down and across sectional view) of the structure of FIGS. 7A-7B after forming agate conductor on the gate dielectric through the extendedsub-lithographic opening.

FIGS. 9A-9B are pictorial representations (through a top-down and across sectional view) of the structure of FIGS. 8A-8B after removing thesacrificial layer from the structure.

FIGS. 10A-10B are pictorial representations (through a top-down and across sectional view) of the structure of FIGS. 9A-9B after forming atleast one spacer on the sidewalls of the gate conductor/gate dielectricstack.

DETAILED DESCRIPTION OF THE INVENTION

The present invention, which provides a semiconductor structureincluding at least one transistor having a sub lithographic gate lengthand a method of fabricating the same, will now be described in greaterdetail by referring to the following description and drawings thataccompany the present application. It is noted that the drawings of thepresent invention are provided for illustrative purposes and, as such,they are not drawn to scale. In each of the drawings, drawing A is atop-down view and drawing B is a cross sectional view through line A-A.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide a thoroughunderstanding of the present invention. However, it will be appreciatedby one of ordinary skill in the art that the invention may be practicedwithout these specific details. In other instances, well-knownstructures or processing steps have not been described in detail inorder to avoid obscuring the invention.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

As stated above, a semiconductor structure having at least onetransistor located on a surface of a semiconductor substrate, whereinthe at least one transistor has a sub-lithographic channel length, isprovided. The present invention also provides a method to form such asemiconductor structure using a self-assembling block copolymer that canbe placed at a specific location using a pre-fabricated hard maskpattern. The inventive method provides a semiconductor structure thathas sub-lithographic channel lengths (e.g., gate lengths less than 60nm) and a line edge roughness that is smoother than that obtained byconventional processing.

Reference is now made to the various drawings which illustrate the basicprocessing steps that are used in the present invention for forming asemiconductor structure including sub-lithographic gate transistors. Theprocessing of the present invention combines conventional lithographictechnology and a copolymer self-assembling technique to place onepolymer unit with precise position. The polymer is self-aligned to thepre-defined lithographically defined pattern boundaries. The singlepolymer can then be selectively removed and the remaining polymer isused as a photo mask to pattern a transistor gate. The final featuresize is determined by the polymer molecule rather than conventionallithography. Because conventional lithography is not used in providingthe final gate transistor, defects such as line edge roughness and CDvariation are avoided.

Reference is made to FIGS. 1A-1B which illustrate an initial structure10 that is employed in the present invention for fabricating thesub-lithographic gate transistors. As is shown, the initial structure 10includes a semiconductor substrate 12, a sacrificial layer 14 located ona surface of the semiconductor substrate 12, and a patterned hard mask16 having at least one lithographically defined mask opening 18 locatedon a surface of the sacrificial layer 14. The at least onelithographically defined mask opening 18 exposes portions of theunderlying sacrificial layer 14.

The structure 10 shown in FIGS. 1A-1B is formed utilizing conventionalmaterials and techniques that are well known to those skilled in theart. For example, the semiconductor substrate 12 is first provided. Thesemiconductor substrate 12 comprises any semiconducting materialincluding, for example, Si, SiGe, SiGeC, SiC, Ge alloys, GaAs, InAs,InP, as well as other III-V or II-VI compound semiconductors. Thesemiconductor substrate 12 may also comprise a layered semiconductorsuch as Si/SiGe, or a semiconductor-on-insulator (SOI). Preferably, thesemiconductor substrate 12 is composed of a Si-containing semiconductormaterial, i.e., a semiconductor material that includes Si. Thesemiconductor substrate 12 may be doped, undoped or contain both dopedand undoped regions therein.

The semiconductor substrate may have a single crystal orientation orhybrid substrates including regions of different crystal orientationsmay be employed. When hybrid substrates are employed, the regions ofdifferent crystal orientations may be co-planar with each other. Hybridsemiconductor structures provide the ability to form the transistor on asurface orientation that provides optimal carrier mobility. For example,and when Si is employed, electron mobility is higher on a {100} surfaceorientation, while hole mobility is higher on a {110} surfaceorientation. In such a case, the {100} Si surface is used as the devicelayer for forming nFETs, while the {110} Si surface is used as thedevice layer for forming pFETs. Formation of such hybrid substrates iswell known to those skilled in the art and, as such, no detailsregarding their formation is provided herein.

In some embodiments of the present invention, at least one isolationregion, such as, for example, a trench isolation or a field oxideisolation region, is formed in the semiconductor substrate 12 to isolatedifferent device regions from each other. Trench isolation regions areformed utilizing a conventional trench isolation process that is wellknown to those skilled in the art. For example, lithography, etching andfilling of the trench with a trench dielectric such as an oxide may beused in forming the same. Optionally, a liner may be formed in thetrench prior to trench fill, a densification step may be performed afterthe trench fill and a planarization process may follow the trench fillas well. Field oxide isolation regions are formed utilizing a so-calledlocal oxidation of silicon process.

After providing the semiconductor substrate 12, a sacrificial layer 14is formed on the surface of the substrate 12. The sacrificial layer 14typically comprises an oxide such as, for example, silicon oxide, orsilicon oxynitride. The sacrificial layer 14 is formed utilizing aconventional deposition process including, for example, chemical vapordeposition (CVD), plasma enhanced chemical vapor deposition (PECVD),evaporation, chemical solution deposition, and atomic layer deposition(ALD). Alternatively, thermal oxidation may be used to form thesacrificial layer 14. The sacrificial layer 14 may also be formedutilizing a combination of any of the aforementioned processes.

The thickness of the as deposited sacrificial layer 14 may varydepending on, for example, the technique used in forming the same.

Following the formation of the sacrificial layer 14, a hard mask layeris formed on the surface of the sacrificial layer 14. The hard masklayer typically comprises a nitride, such as, for example, siliconnitride. The hard mask layer is formed utilizing a conventionaldeposition process and/or thermal nitrification. The thickness of thehard mask layer may vary depending on the technique used in forming thesame. Typically, the hard mask layer has a thickness from about 20 toabout 60 nm, with a thickness from about 30 to about 50 nm being evenmore typical.

After forming the hard mask layer on the surface of the sacrificiallayer, the hard mask layer is patterned to form patterned hard mask 16which has at least one lithographically defined mask opening 18 therein.The patterned hard mask 16 is formed by applying a photoresist (notshown) to the hard mask layer, exposing the photoresist to a pattern ofradiation, developing the exposed resist material to provide a patternedresist atop of the hard mask layer, and thereafter transferring thepattern into the hard mask layer by etching. A dry etching process(including reactive ion etching, ion beam etching and plasma etching) ora wet chemical etching process may be used to pattern the hard masklayer.

The width (W) of the lithographically defined mask opening 18 inpatterned hard mask 16 is limited by the minimum printing criticaldimension (CD) of conventional lithography and cannot be further reducedusing the conventional lithographic techniques alone. Typically, thewidth W is about 80 nm or greater. The present invention thereforeemploys self-assembling block copolymers to form sub-lithographicpatterns in the lithographically defined mask opening 18.

It has been known that certain materials are capable of spontaneousorganization of materials into ordered patterns without the need forhuman interference, which is typically referred to as the self-assemblyof materials. Examples of material self-assembly range from snowflakesto seashells to sand dunes, all of which form some type of regular orordered patterns in response to the external conditions.

Among various self-assembling materials, self-assembling blockcopolymers that are capable of self-organizing into nanometer-scalepatterns are particularly promising for enabling future advances in thesemiconductor technology. Each self-assembling block copolymer systemtypically contains two or more different polymeric block components thatare immiscible with one another. Under suitable conditions, the two ormore immiscible polymeric block components separate into two or moredifferent phases on a nanometer scale and thereby form ordered patternsof isolated nano-sized structural units.

Such ordered patterns of isolated nano-sized structural units formed bythe self-assembling block copolymers can be used for fabricatingnano-scale structural units in semiconductor, optical, and magneticdevices. Specifically, dimensions of the structural units so formed aretypically in the range of 10 to 40 nm, which are sub-lithographic (i.e.,below the resolutions of the lithographic tools). Further, theself-assembling block copolymers are compatible with conventionalsemiconductor, optical, and magnetic processes. Therefore, the orderedpatterns of nano-sized structural units formed by such block copolymershave been integrated into semiconductor, optical, and magnetic deviceswhere a large, ordered array of repeating structural units are required.

However, the CMOS technology requires precise placement or registrationof individual structural units for formation of semiconductor devices.Therefore, the large, ordered array of repeating structural units formedby self-assembling block copolymers could not be used in CMOS devices,because of lack of alignment or registration of the position ofindividual structure unit.

The present invention provides a method that combines a conventionallithographic technology with the self-assembling block copolymertechnology to form sub-lithographic patterns within the lithographicallydefined mask opening 18.

Specifically, a thin layer of a self-assembling block copolymer (havinga thickness typically ranging from about 20 nm to about 100 nm) is firstapplied over the structure shown in FIGS. 1A-1B and then annealed toform an ordered pattern containing repeating structural units inside thealready formed lithographically defined mask opening 18.

FIGS. 2A-2B show the structure after applying the thin self-assemblyblock copolymer 20 to the structure shown in FIGS. 1A-1B, while FIGS.3A-3B show the structure after annealing which forms an annealed blockcopolymer 20′.

It is noted that the width (W) of the lithographically defined maskopening 18 is carefully adjusted so that only a single unit polymerblock can be formed therein from the self-assembling block copolymer.The single unit polymer block is embedded in a polymeric matrix that isin turn located inside the lithographically defined mask opening 18, andit has a width that is less than the diameter of the lithographicallydefined mask opening 18. The single unit polymer block can beselectively removed relative to the polymeric matrix, thereby leaving asingle opening of the smaller width in the polymeric matrix inside thelithographically defined mask opening 18′. The single opening of thesmaller width can then be used for sub-lithographic gate transistors.

There are many different types of block copolymers that can be used forpracticing the present invention. As long as a block copolymer containstwo or more different polymeric block components that are not immisciblewith one another, such two or more different polymeric block componentsare capable of separating into two or more different phases on ananometer scale and thereby form patterns of isolated nano-sizedstructural units under suitable conditions.

In a preferred, but not necessary, embodiment of the present invention,the block copolymer consists essentially of a first polymeric component(A) and a second polymeric block component (B) that are immiscible witheach other. The block copolymer may contain any numbers of the polymericblock components A and B arranged in any manner. The block copolymer canhave either a linear or a branched structure. Preferably, such a blockpolymer is a linear diblock copolymer having the formula of A-B.

Specific examples of suitable block copolymers that can be used forforming the structural units of the present invention may include, butare not limited to: polystyrene-block-polymethylmethacrylate(PS-b-PMMA), polystyrene-block-polyisoprene (PS-b-PI),polystyrene-block-polybutadiene (PS-b-PED),polystyrene-block-polyvinylpyridine (PS-b-PVP),polystyrene-block-polyethyleneoxide (PS-b-PEO),polystyrene-block-polyethylene (PS-b-PE),polystyrene-b-polyorganosilicate (PS-b-POS),polystyrene-block-polyferrocenyldimethylsilane (PS-b-PFS),polyethyleneoxide-block-polyisoprene (PEO-b-PI),polyethyleneoxide-block-polybutadiene (PEO-b-PBD),polyethyleneoxide-block-polymethylmethacrylate (PEO-b-PMMA),polyethyleneoxide-block-polyethylethyl (PEO-b-PEE),polybutadiene-block-polyvinylpyridine (PBD-b-PVP), andpolyisoprene-block-polymethylmethacrylate (PI-b-PMMA).

The specific structural units formed by the block copolymer aredetermined by the molecular weight ratio between the first and secondpolymeric block components A and B. For example, when the ratio of themolecular weight of the first polymeric block component A over themolecular weight of the second polymeric block component B is greaterthan about 80:20, the block copolymer will form an ordered array ofspheres composed of the second polymeric block component B in a matrixcomposed of the first polymeric block component A. When the ratio of themolecular weight of the first polymeric block component A over themolecular weight of the second polymeric block component B is less thanabout 80:20 but greater than about 60:40, the block copolymer will forman ordered array of cylinders composed of the second polymeric blockcomponent B in a matrix composed of the first polymeric block componentA. When the ratio of the molecular weight of the first polymeric blockcomponent A over the molecular weight of the second polymeric blockcomponent B is less than about 60:40 but is greater than about 40:60,the block copolymer will form alternating lamellae composed of the firstand second polymeric block components A and B. Therefore, the molecularweight ratio between the first and second polymeric block components Aand B can be readily adjusted in the block copolymer of the presentinvention, in order to form desired structural units.

In a preferred embodiment of the present invention, the ratio of themolecular weight of the first polymeric block component A over themolecular weight of the second polymeric block component B ranges fromabout 60:40 to about 80:20, so that the block copolymer of the presentinvention will form alternating layers of the first polymeric blockcomponent A and the second polymeric block component B.

Preferably, one of the components A and B can be selectively removablerelative to the other, thereby resulting in either orderly arrangedstructural units composed of the un-removed component. For example, whenthe second polymeric block component B is selectively removable relativeto the first polymeric block component A, orderly arranged trenches canbe formed.

In a particularly preferred embodiment of the present invention, theblock polymer used for forming the self-assembled periodic patterns ofthe present invention is PS-b-PMMA with a PS:PMMA molecular weight ratioranging from about 20:80 to about 80:20.

Typically, mutual repulsion between different polymeric block componentsin a block copolymer is characterized by the term _(X)N, where _(X) isthe Flory-Huggins interaction parameter and N is the degree ofpolymerization. The higher _(X)N, the higher the repulsion between thedifferent blocks in the block polymer, and the more likely the phaseseparation therebetween. When _(X)N>10 (which is hereinafter referred toas the strong segregation limit), there is a strong tendency for thephase separation to occur between different blocks in the block polymer.

For a PS-b-PMMA diblock copolymer, _(X) can be calculated asapproximately 0.028+3.9/T, where T is the absolute temperature.Therefore, _(X) is approximately 0.0362 at 473K (≈200° C.). When themolecular weight (M_(n)) of the PS-b-PMMA diblock copolymer isapproximately 64 Kg/mol, with a molecular weight ratio (PS:PMMA) ofapproximately 66:34, the degree of polymerization N is about 622.9, so_(X)N is approximately 22.5 at 200° C.

In this manner, by adjusting one or more parameters such as thecomposition, the total molecular weight, and the annealing temperature,the mutual compulsion between the different polymeric block componentsin the block copolymer of the present invention can be readilycontrolled to effectuate desired phase separation between the differentblock components. The phase separation in turn leads to formation ofself-assembled periodic patterns containing ordered arrays of repeatingstructural units (i.e., spheres, cylinders, or lamellae), as describedhereinabove.

In order to form the self-assembled periodic patterns, the blockcopolymer is first dissolved in a suitable solvent system to form ablock copolymer solution, which is then applied onto a surface to form athin block copolymer layer, followed by annealing of the thin blockcopolymer layer, thereby effectuating phase separation between differentpolymeric block components contained in the block copolymer.

The solvent system used for dissolving the block copolymer and formingthe block copolymer solution may comprise any suitable solvent,including, but not limited to: toluene, propylene glycol monomethylether acetate (PGMEA), propylene glycol monomethyl ether (PGME), andacetone. The block copolymer solution preferably contains the blockcopolymer at a concentration ranging from about 0.1% to about 2% bytotal weight of the solution. More preferably, the block copolymersolution contains the block copolymer at a concentration ranging fromabout 0.5 wt % to about 1.5 wt %.

The block copolymer solution can be applied to the surface of astructure by any suitable techniques, including, but not limited to:spin casting, coating, spraying, ink coating, dip coating, etc.Preferably, the block copolymer solution is spin cast onto the surfaceof a structure to form a thin block copolymer layer thereon.

After application of the thin block copolymer layer onto theinterconnect surface such as is shown, for example, in FIGS. 2A-2B, theentire structure is annealed to effectuate micro-phase segregation ofthe different block components contained by the block copolymer, therebyforming the periodic patterns with repeating structural units. FIGS.3A-3B shows the structure after annealing. Annealing of the blockpolymer in the present invention can be achieved by various methodsknown in the art, including, but not limited to: thermal annealing(either in a vacuum or in an inert atmosphere containing nitrogen orargon), ultra-violet annealing, laser annealing, solvent vapor-assistedannealing (either at or above room temperature), and supercriticalfluid-assisted annealing, which are not described in detail here inorder to avoid obscuring the invention.

In a particularly preferred embodiment of the present invention, athermal annealing step is carried out to anneal the block copolymerlayer at an elevated annealing temperature that is above the glasstransition temperature (T_(g)) of the block copolymer, but below thedecomposition or degradation temperature (T_(d)) of the block copolymer.More preferably, the thermal annealing step is carried out an annealingtemperature of about 200° C.-300° C. The thermal annealing may last fromless than about 1 hour to about 100 hours, and more typically from about1 hour to about 15 hours. In an alternative embodiment of the presentinvention, the block copolymer layer is annealed by ultra-violet (UV)treatment.

It is noted that the annealed block copolymer 20′ can comprise a firstpolymeric component A and a second polymeric component B, in which thepattern is an ordered array of cylinders composed of the block componentB in a polymeric matrix composed of the polymeric block component A.Such an annealed block copolymer in which the ordered array of cylindersis aligned vertical with a surface of the sacrificial layer 14 is shownin FIGS. 3A-3B. It is noted in the embodiment illustrated the B blockscan be used to provide sub-lithographic openings having a width fromabout 10 to about 40 nm into the sacrificial layer 14. This is achievedby having the cylindrical shaped block aligned vertical to the surfaceof the sacrificial layer 14. The particular alignment of the cylindricalblock is determined by the surface wetting conditions as are describedin greater detail below.

The present invention employs lithographic features to limit formationand placement of the structural units formed by the self-assemblingblock copolymer materials. More specifically, the lithographic featuresas used by the present invention have dimensions that are adjusted sothat only a single unit polymer block can be formed and placed insideeach lithographic feature from the self-assembling block copolymer.

Optionally, but not necessarily, the interior surfaces of thelithographically defined mask opening 18 are treated before applicationof a block copolymer layer. Specifically, one or more surface layers areformed over the bottom surface and sidewall surfaces of thelithographically defined mask opening 18, so as to provide the desiredwetting properties for aligning the alternating layers of unit polymericblocks to be formed with the lithographically defined mask opening 18.

The wetting properties as discussed herein refer to the surfaceaffinities of a specific surface with respect to the different blockcomponents of the block copolymers. For example, if a surface hassubstantially the same surface affinity to both block components A and Bof a block copolymer, such a surface is considered a neutral surface ora non-preferential surface, i.e., both block components A and B can wetor have affinities to such a surface. In contrast, if a surface hassignificantly different surface affinities for the block components Aand B, such a surface is then considered a preferential surface, i.e.,only one of block components A and B can wet such a surface, but theother cannot.

Surfaces comprising one of silicon native oxides, silicon oxides, andsilicon nitrides are preferentially wetted by PMMA block components, butnot by PS block components. Therefore, such surfaces can be used aspreferential surfaces for PS-b-PMMA block copolymers. On the other hand,a monolayer comprising a substantially homogenous mixture of PS and PMMAcomponents, such as a random PS-r-PMMA copolymer layer, provides aneutral surface or a non-preferential surface for PS-b-PMMA blockcopolymers.

In order to form alternating layers of polymeric blocks that are alignedperpendicular to the bottom surface of the lithographically defined maskopening 18 from PS-b-PMMA, it is desired to deposit a neutral ornon-preferential monolayer (e.g., a substantially homogenous mixture ofPS and PMMA components) over the bottom surface of the lithographicallydefined mask opening 18, while the sidewall surfaces of thelithographically defined mask opening 18 are either left untreated orare coated with a preferential wetting material (e.g., silicon nativeoxides, silicon oxides, and silicon nitrides). In this manner, thealternating layers of polymeric blocks formed from PS-b-PMMA will standperpendicular to the bottom surface of the lithographic mask opening 18.

FIGS. 4A-4B show the structure after etching the second polymeric blockcopolymer component B therefrom. As shown, the resultant structure nowincludes a sub-lithographic opening 22 having a width that is less than60 nm, preferably less than about 50 nm, in the annealed block copolymer20′. The sub-lithographic opening 22 is formed by selectively etching(via a dry etching or wetting etching process) the second polymericblock copolymer component B relative to the first polymeric blockcopolymer component A.

FIGS. 5A-5B show the structure after extending the sub-lithographicopening 22 though portions of the underlying sacrificial layer 14,stopping atop the upper surface of substrate 12. The extending of thesub-lithographic opening 22 is performed utilizing an etching processsuch as, for example, reactive-ion etching.

FIGS. 6A-6B show the structure that is formed after removing theremaining block copolymer and patterned hard mask 16 from the structure.The remaining block copolymer may be removed utilizing a conventionalstripping process well known to those skilled in the art. The patternedhard mask 16 may be removed by a conventional planarization process suchas, for example, chemical mechanical polishing (CMP) and/or grinding.The planarization process stops on an upper surface of the underlyingsacrificial layer 14 which includes the sub-lithographic opening 22therein.

FIGS. 7A-7B illustrate the structure after forming a gate dielectric 24on the exposed surface of the semiconductor substrate 12 through thesub-lithographic opening 22. Prior to formation of the gate dielectric24, an ion implantation step may be performed to dope the channel regionn-type or p-type. Where appropriate, block masks can be used to preventunwanted doping in exposed portions of the semiconductor substrate 12.Such an embodiment can be used when nFETs and pFETs are formed.

The gate dielectric 24 comprises any suitable dielectric materialincluding, but not limited to oxides, nitrides, oxynitrides, silicates(e.g., metal silicates and nitrided metal silicates) and multilayersthereof. In one embodiment, it is preferred that the gate dielectric 24comprise an oxide such as, for example, SiO₂, HfO₂, ZrO₂, Al₂O₃, TiO₂,La₂O₃, SrTiO₃, LaAlO₃ and multilayers thereof. The physical thickness ofthe gate dielectric 24 may vary depending on the specific technique usedin forming the same. Typically, the gate dielectric 24 has a thicknessfrom about 0.5 to about 10 nm, with a thickness from about 1 to about 7nm being even more typical.

The gate dielectric 24 may be formed by a conventional depositionprocess such as, for example, CVD, PECVD, ALD, evaporation, reactivesputtering and chemical solution deposition. A thermal process such asan oxidation, nitrification or oxynitridation may also be employed. Thegate dielectric 24 may also be formed utilizing a combination of theabove-mentioned techniques.

FIGS. 8A-8B shows the structure after filling the remaining portions ofthe sub-lithographic opening 22 with a gate conductor 26 andplanarization. The gate conductor 26 may comprise any suitableconductive material including, but not limited to doped polySi, dopedSiGe, elemental metals, alloys of elemental metals, metal silicides andmultilayers thereof. The gate conductor 26 is formed utilizingconventional techniques well known to those skilled in the art. Afterdeposition of the conductive material, a conventional planarizationprocess such as CMP and/or grinding is used.

FIGS. 9A-9B show the resultant structure that is formed after removingthe sacrificial layer 14 therefrom. Any conventional etch back processthat selectively removes the sacrificial layer 14 can be used in thepresent invention.

FIGS. 10A-10B shows the structure after formation of at least one spaceron sidewalls of the gate conductor/gate dielectric stack. The at leastone spacer may include a single spacer or a pair of spacers, as is shownin FIGS. 10A-10B. The pair of spacers includes an inner spacer 28 and anouter spacer 30. The spacers are formed utilizing conventionalprocessing well known to those skilled in the art. Extension implantsare typically performed after formation of the inner spacer, andsource/drain diffusion regions are formed after outer spacer formation.The implants used in forming the extension region (not specificallyshown) and the source/drain diffusions (not specifically show) arecarried out be conventional ion implantation. An activation anneal isused to active the implanted dopant regions.

At this point of the present invention further CMOS and/or interconnectprocessing may be performed. For example, a stress liner may be formedto stress the channel region, and silicide contacts may be formed atopthe source/drain diffusions and optionally, the gate conductor (ifcomposed of polySi or SiGe).

Because of the method of the present invention, the final feature of thetransistor including the channel length is than 60 nm. It is noted thatthe processing steps mentioned above can be used to form a plurality ofsub-lithographic gate transistors which may or may not be of the sameconductive type. It is also noted that the line edge roughness of thetransistor formed by the inventive method is smoother than that obtainedby conventional techniques.

While the present invention has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present invention. It is therefore intended that the presentinvention not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1. A method of fabricating a semiconductor structure having asub-lithographic channel length comprising: providing a structureincluding a patterned hard mask located above a sacrificial layer and asemiconductor substrate, wherein said patterned hard mask includes atleast one lithographically defined opening exposing a surface of saidsacrificial layer; providing a block copolymer having a single unitpolymer block with a sub-lithographic width inside the at least onelithographically defined opening, wherein the single unit polymer blockcomprises a second polymeric block component which is embedded in apolymeric matrix that comprises a first polymeric block component ofsaid block copolymer; selectively removing the second polymeric blockcomponent relative to the first polymeric block component to form asub-lithographic opening in the polymeric matrix inside the at least onelithographically defined opening; transferring said sub-lithographicopening to said sacrificial layer; removing the block copolymer and thepatterned hard mask; forming a gate dielectric and a gate conductorwithin said at least one lithographical defined opening; and removingthe sacrificial layer.
 2. The method of claim 1 wherein said blockcopolymer is selected from the group consisting ofpolystyrene-block-polymethylmethacrylate (PS-b-PMMA),polystyrene-block-polyisoprene (PS-b-PI),polystyrene-block-polybutadiene (PS-b-PBD),polystyrene-block-polyvinylpyridine (PS-b-PVP),polystyrene-block-polyethyleneoxide (PS-b-PEO),polystyrene-block-polyethylene (PS-b-PE),polystyrene-b-polyorganosilicate (PS-b-POS),polystyrene-block-polyferrocenyldimethylsilane (PS-b-PFS),polyethyleneoxide-block-polyisoprene (PEO-b-PI),polyethyleneoxide-block-polybutadiene (PEO-b-PBD),polyethyleneoxide-block-polymethylmethacrylate (PEO-b-PMMA),polyethyleneoxide-block-polyethylethylene (PEO-b-PEE),polybutadiene-block-polyvinylpyridine (PBD-b-PVP), andpolyisoprene-block-polymethylmethacrylate (PI-b-PMMA).
 3. The method ofclaim 2 wherein the block copolymer comprises PS-b-PMMA having a PS:PMMAweight ratio ranging from about 20:80 to about 80:20.
 4. The method ofclaim 1 wherein said second polymeric copolymer component of the blockcopolymer comprises an ordered array of cylinders that is alignedvertical to a surface of said semiconductor substrate.
 5. The method ofclaim 1 wherein said providing said block copolymer includes depositionand annealing.
 6. The method of claim 5 wherein said annealing includesat least one of thermal annealing, ultra-violet annealing, laserannealing, solvent vapor-assisted annealing and supercriticalfluid-assisted annealing.
 7. The method of claim 1 wherein saidsub-lithographic opening has a width of less than 60 nm.
 8. The methodof claim 1 wherein said gate dielectric and said gate conductor have afeature size of less than 60 nm and a line edge roughness in terms of3-sigma of less than 6 nm.